Surveying Methods

Generally, the surveyed area extended across the active channel from bank to bank, just like the previous studies on the Missouri and Mississippi Rivers (Huizinga and others, 2010; Huizinga, 2012, 2013; also refer to references listed in table 1). The surveyed reaches were about 1,640 ft long in the direction of flow, positioned so that the surveyed highway bridges were about one-third to one-half of the total length from the upstream boundary, generally using the same upstream and downstream boundaries as were used in the 2011 flood study on the Missouri River (Huizinga, 2012) and in the earliest Mississippi River surveys (Huizinga, 2015). The upstream and downstream boundaries of the surveyed areas were assumed to capture all the substantial hydraulic effects (wake vortices and shear flow) of the bridge structures.

Like in previous studies, bathymetric data were obtained along longitudinal transect lines, and each survey was designed such that the survey swaths overlapped to ensure complete coverage of the channel bed and minimize sonic “shadows” (Huizinga and others, 2010). Many of the surveyed swaths had substantial overlap (ranging from 25 to 50 percent), except in shallow areas near the channel banks or spur dikes and near in-flow structures or debris rafts (overlap ranging from 0 to 10 percent). Areas near bridge piers and along the banks also were surveyed in an upstream direction with the MBES swath electronically tilted to port or starboard to increase the acquisition of bathymetric data higher on the banks and sides of the piers. The electronically tilted swath generally was 120 to 160 degrees wide, extending from 10 degrees above horizontal on the bank-ward or pier-ward side of the survey vessel, and 20 to 60 degrees past nadir below the vessel.

After completing the bathymetric survey at a given site, velocity data were obtained with the ADCP on seven transections spanning the channel within the study area. The position and speed of the boat was determined using a differential GNSS receiver mounted on a pole directly above the ADCP. The bottom-track reference method for determining boat speed was anticipated to be unusable because of moving channel-bed material, so the boat velocity was determined using the GNSS essential fix data from the differential GNSS receiver (the NMEA–0183 GGA string [shorthand for the $GPGGA standard output format for GNSS essential fix data defined by the National Marine Electronics Association 0183 standard that includes information on the three-dimensional location and accuracy of the GNSS receiver; National Marine Electronics Association, 2002]). The distance between the velocity section lines generally was about 260 ft. Three sections were upstream, and four sections were downstream from the bridge being surveyed. Each section line was traversed in each direction across the river, and the reported velocity values were the average from the two traverses of a given section line. Streamflow for a site was computed as the average of the streamflow measurements from the reciprocal pairs (two transects per section line) at all the various sections in the reach. Generally, measured streamflow for any individual transect was within 5 percent of the average.

Survey Quality-Assurance Measures

For the MBMS, the principal quality-assurance measures were assessed in real time during the survey. The MBMS operator assessed the quality of the collected data in real time by making visual observations of cross-track swath shape (such as convex, concave, or skewed bed returns in flat, smooth bottoms), noting data-quality flags and alarms from the MBES and the INS, and noting comparisons between adjacent overlapping swaths.

In addition to the real-time quality-assurance assessments, beam angle checks and a suite of patch tests were done to ensure quality data were acquired from the MBMS for the 2022 surveys. Patch tests are a series of dynamic calibration tests that are used to check for subtle variations in the orientation and timing of the MBES with respect to the INS and real-world coordinates (Huizinga, 2023), and primarily are used to determine angular offsets to roll, pitch, and yaw caused by the alignment of the transducer head relative to the INS (fig. 4). These offsets have been observed to be essentially constant for a given equipment configuration and survey season, barring an event that causes the mount to change such as striking a floating or submerged object (refer to report references listed in table 1; Huizinga and others, 2023; Rivers and others, 2023).

The beam angle check and an initial patch test were completed on April 13, 2022, at Fellows Lake near Springfield, Mo. (fig. 1). An additional patch test was completed during surveying of the Calumet Sag channel in Chicago, Ill. (not shown on fig. 1), on August 3, 2022. The results of the beam angle check (table 4) were within the recommended performance standards used by the U.S. Army Corps of Engineers for hydrographic surveys for all the representative angles below 75 degrees (U.S. Army Corps of Engineers, 2013). Points acquired outside of the central 100–110 degrees of the swath generally had overlap with adjacent swaths, which increases the quality of the survey in the overlapped areas because of duplication.

Although the MBES had several minor strikes of floating or submerged debris at various times during the 2022 survey season, no changes to the roll or pitch angles were apparent from the beginning to the end of the river surveys in May (table 5). The tightly coupled configuration of the Norbit iWBMSh, wherein the IMU of the INS is mounted on the same mounting bracket as the MBES (fig. 3A) results in no measured timing offset and no measured angular offset for pitch (table 5). The yaw is a measure of the alignment of the GNSS receivers relative to the IMU of the INS on the echosounder head, and no offset for yaw was measured in either patch test (table 5). The measured angular offset for roll remained a constant −0.10 (table 5), which is consistent with results for this equipment configuration in other recent surveys (Huizinga, 2023; Rivers and others, 2023). It was noted in the earliest work with the MBMS in Missouri (Huizinga, 2010) that a sensitivity analysis of the four offsets indicated that the ultimate position of surveyed points in three-dimensional space was least sensitive to the angular offset for yaw, whereas it was most sensitive to the angular offset for roll. Processing all the data for the bridge surveys detailed in this report with an angular offset of roll of –0.10 degree and no angular offset for pitch or yaw generally yielded good results with no noticeable artifacts caused by incorrect offsets.

Angular offsets were applied to the bathymetric data before removing data spikes and other spurious points in the multibeam swaths using automatic filters and manual editing. The bathymetric data were then projected to a three-dimensional grid at a resolution of 1.64 ft using the Combined Uncertainty and Bathymetry Estimator (CUBE) method (Calder and Mayer, 2003), as implemented in the MBMax processing package of the HYPACK/HYSWEEP software (HYPACK, Inc., 2020) and used to generate a gridded raster surface of the channel bed (and associated uncertainty) near each bridge (hereinafter referred to as a “bathymetric surface”) using ArcMap (version 10.8.1; Esri, 2023).

A quality-assurance plan has been established for streamflow and velocity measurements using ADCPs that includes several instrument diagnostics checks and calibrations. These standard operating procedures were followed when acquiring the velocity profile data for these surveys. For a detailed description of these procedures, refer to Mueller and others (2013).

Uncertainty Estimation

Similar to recent bathymetry studies in Missouri (refer to report references listed in table 1; Huizinga and others, 2023; Rivers and others, 2023), uncertainty in each survey was estimated by computing the uncertainty for each survey-grid cell in the bathymetric surface of the survey area using the CUBE method (Calder and Mayer, 2003). The gridded uncertainty is a measure of the variability of the individual points in the cell used to determine the CUBE-derived elevation for the cell. Statistics of gridded uncertainty for each of the survey areas are shown in table 6. An example of the spatial distribution of gridded uncertainty typically observed in the survey data at structure A1700 on Interstate 155 is shown in figure 5. The uncertainty data were output and combined with the three-dimensional bathymetry data and are included with metadata in the USGS data release associated with this study (Huizinga and Rivers, 2023b).

More than 90 percent of the uncertainty values at all the sites were less than 0.50 ft (table 6), which is within the specifications for a “Special Order” survey, the second-most-stringent survey standard of the International Hydrographic Organization (IHO; International Hydrographic Organization, 2020). For all but structure A1700 (site 38), more than 75 percent of the uncertainty values were less than 0.25 ft (table 6). The tops of bridge substructural elements (pier footings and seal courses) typically had uncertainty values of less than 0.25 ft. Like noted in previous surveys with this type of equipment (for example, Huizinga, 2012, 2016), the uncertainty values were larger near moderate-relief features (banks, spur dikes, rock riprap and outcrops, and scour holes near piers). The largest uncertainty in this group of surveys was 6.56 ft (table 6); however, as noted in previous studies, uncertainty values of this magnitude typically happened near high-relief features, such as the front or side of a pier footing. Occasionally, the uncertainty values also were larger (1.00 ft or greater) in the outermost beams of the multibeam swath where overlap occurred with an adjacent swath, particularly when the MBES head was electronically tilted for the survey lines along the banks. Overlapping adjacent swaths in the channel thalweg (the line of maximum depth in the channel) also can display larger uncertainty values because substantial bed movement can happen between survey passes (fig. 5).

The uncertainty of the gridded data computed using the CUBE method has been decreasing with time compared to previous surveys (refer to table 4 in previous studies of the periphery of Missouri area; Huizinga, 2012, 2015, 2020e). The decrease in uncertainty primarily is the result of improvements in data-collection equipment and methods. The tightly coupled configuration of the Norbit iWBMSh used in these surveys decreases some of the uncertainty by substantially reducing the lever arm length (and therefore the potential movement) between the MBES and IMU. The ability to electronically tilt the swath substantially reduces the time between when unrotated, down-looking data and rotated, side-looking data are collected, which reduces the uncertainty of the data in the swath overlap zone that might otherwise experience more substantial bed movement.

The survey at structure A1700 on Interstate 155 near Caruthersville had one of the highest maximum values (6.56 ft) and the highest median value (0.26 ft) of gridded uncertainty, as well as a lower percentage of bathymetry points with a gridded uncertainty of less than the various thresholds (table 6). The survey at this site was obtained with relatively smooth longitudinal swaths (fig. 5), which was the case at nearly all the sites surveyed in this study. The primary anomalies at this site were observed near the outermost beams of adjacent swaths and along the banks and near the piers, where the MBES was used in an electronically tilted configuration to extend the potential coverage in these areas, resulting in higher uncertainty values. Generally, the magnitude and distribution of uncertainty observed at this site are representative of those observed at all the sites.

Structure L0098 on U.S. Highway 136 at Brownville, Nebraska

Structure L0098 (site 1; table 2) on U.S. Highway 136 crosses the Missouri River at river mile (RM) 535.3 at Brownville, Nebr., in the northwestern corner of Missouri (fig. 1). The site was surveyed on June 22, 2022, and the average water-surface elevation of the river in the survey area, determined by the RTK GNSS tide solution, was 884.3 ft (table 3; fig. 7). Streamflow on the Missouri River was about 37,300 ft³/s during the survey (table 3).

The survey area was about 1,640 ft long and about 740 ft wide, extending across the active channel from bank to bank, except for a shallow area between the spur dikes on the left (eastern) bank (fig. 7). The survey area extended about 740 ft upstream from the centerline of structure L0098. The channel-bed elevations ranged from about 864 to 876 ft for most of the surveyed area (5th to 95th percentile range of the bathymetric data; fig. 8), on the downstream sides of the various spur dikes on the left (east) bank (fig. 7; table 3). The channel bed was covered with small to medium dunes and ripples. As in previous surveys (Huizinga, 2012, 2015, 2020e), a rock outcrop was present on the right (west) bank upstream from the bridge, and a stone revetment was present on the right bank downstream from the bridge (fig. 7).

The scour hole that historically had been observed near pier 3 in previous surveys (refer to fig. 6 in Huizinga [2020e]) was conspicuously absent in the 2022 survey, having been mitigated by the apparent installation of a riprap blanket scour countermeasure around the pier sometime since the most-recent previous survey in 2018 (figs. 7, 1.1A, 1.1B). Information from bridge plans indicates that pier 3 is founded on sheet piling caissons on bedrock, with about 51 ft of bed material between the bottom of the scour hole and bedrock at the upstream face of the pier (fig. 9; difference between “Approximate minimum elevation in scour hole near pier/bent” and “Approximate elevation of bedrock near pier/bent” in table 7). The unique configuration of the sheet pile caissons and ice breaker of the pier are clearly seen in the shaded TIN images for pier 3 (fig. 1.1). The surveyed bed generally was similar to the most-recent previous multibeam surveys in 2014 and 2018, except near pier 3 where the riprap blanket scour countermeasures now exist (fig. 9).

The computed difference between the survey DEMs with a probabilistic threshold mask of 95-percent confidence based on uncertainty (hereinafter, referred to simply as “the difference”) from the survey on June 22, 2022, and the previous survey on August 13, 2018 (fig. 10), indicated about 93 percent of the joint area of interest had change greater than the 95-percent confidence interval of uncertainty (hereinafter referred to as “detectable change”), which means only about 7 percent of the differences in the joint area of interest were equivocal and within the bounds of uncertainty (table 8). Bed variation seemed about equal between scour and deposition from 2018 to 2022 in the DoD (fig. 10); however, erosion was dominant in the upstream part of the reach and along the tips of the spur dikes on the left (east) bank throughout the reach, whereas substantial deposition was apparent near the pier, in the wake vortex area downstream, and between the spur dikes on the left side of the channel near the left (east) bank (fig. 10). The average difference between the bathymetric surfaces (the statistical mean value of the gridded raster surface [fig. 10] created from the thresholded difference between the 2022 and 2018 gridded raster bathymetric surfaces) was +0.41 ft (table 8), indicating minor to moderate channel aggradation between the 2018 and 2022 surveys. The net volume of cut in the reach from 2018 to 2022 was about 34,500 cubic yards (yd³), and the net volume of fill was about 50,100 yd³, resulting in a net gain of about 15,600 yd³ of sediment between 2018 and 2022 (table 8). The cross sections from the two surveys along the upstream face of the bridge are not substantially different from one another for most of the cross section, except pier 3 (fig. 9). The frequency distribution of bed elevations in 2022 also was similar to 2018 and 2014 (fig. 8). The stone revetement on the right (west) bank showed areas of minor scour and deposition, and the spur dike on the left (east) bank showed minor erosion on one side and deposition on the other (fig. 10); however, deposition or scour apparent on opposing faces of a feature likely resulted from minor horizontal positional variances between the surveys (fig. 6).

The difference between the survey on June 22, 2022, and the survey on June 3, 2014, (fig. 11), indicates about 57 percent of the joint area of interest had detectable change, which means about 43 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Scour and deposition are roughly balanced throughout most of the reach between 2014 and 2022 in the DoD, except along the left (east) bank near the spur dikes, and near pier 3 (fig. 11). The average difference between the bathymetric surfaces was +0.27 ft (table 8), indicating minor overall channel aggradation between the 2014 and 2022 surveys. The net loss of sediment between 2014 and 2022 was about 19,700 yd³, and the net volume of fill was about 26,000 yd³, resulting in a net gain of only about 6,300 yd³ of sediment between 2018 and 2022 (table 8). Streamflow in 2022 was very similar to 2014 (table 8), which may account for the similarity in overall characteristics between the surveys (figs. 9 and 11). The stone revetment and rock outcrop on the right (west) bank has localized areas of deposition (fig. 11). As with the previous DoD, deposition or scour apparent on opposing faces of a feature (such as a pier or spur dike) likely results from minor horizontal positional variances between the surveys (fig. 6).

The difference between the survey on June 22, 2022, and the earliest survey during flooding on July 13, 2011 (fig. 12), indicates about 92 percent of the joint area of interest had detectable change, which means only about 8 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Deposition is dominant throughout most of the reach between 2011 and 2022 in the DoD, except in localized areas near the tips of the spur dikes on the left (east) bank (fig. 12). The average difference between the bathymetric surfaces was +4.43 ft (table 8), indicating substantial channel aggradation between the 2011 and 2022 surveys. The net gain of sediment in the reach between 2011 and 2022 was about 165,100 yd³, which is by far the largest net gain of the two sites on the Missouri River, as well as the second largest net gain of all the sites surveyed in 2022 (table 8). The cross section from the 2011 survey along the upstream face of the bridge varies from 5 to 15 ft below the other survey sections, with a substantially lower bed near the left (east) bank downstream from the spur dike there (fig. 9). The frequency distribution of bed elevations in 2011 is unique compared to the other distributions at this site, with a substantially higher percentage of cells at channel-bed elevations 8–10 ft lower than the other surveys (fig. 8). The stone revetment on the right (west) bank showed signs of localized substantial deposition, whereas the spur dikes on the left (east) bank showed minor scour (fig. 12). As with the previous DoD, deposition or scour apparent on opposing faces of a feature (such as a pier or spur dike) likely resulted from minor horizontal positional variances between the surveys (fig. 6).

The vertically averaged velocity vectors indicated mostly uniform flow in the middle of the channel, with velocities ranging from about 4 to 7 feet per second (ft/s) except near the spur dikes on the left (east) bank where local lower velocities and turbulence were observed (fig. 13). The wake vortices downstream from the main channel pier were not pronounced and seemed to be no greater than the general nonuniformity of flow observed in the channel (fig. 13). Minor localized turbulence was present in all the sections (fig. 13).

Dual Bridge Structure A3664 on U.S. Highway 36 at St. Joseph, Missouri

Structure A3664 (site 2; table 2) consists of twin bridges on U.S. Highway 36 crossing the Missouri River at RM 447.9 at St. Joseph, Mo., on the western side of Missouri north of Kansas City, Mo. (fig. 1). The site was surveyed on June 22, 2022, and the average water-surface elevation of the river in the survey area, determined by the RTK GNSS tide solution, was 796.1 ft (table 3; fig. 14) and streamflow was about 38,700 ft³/s during the survey (table 3).

The survey area was about 1,640 ft long and about 885 ft wide, extending from bank to bank in the main channel (fig. 14). The survey area extended about 655 ft upstream from the centerline of structure A3664 (fig. 14). The channel-bed elevations ranged from about 770 to 789 ft throughout the survey area (5th to 95th percentile range of the bathymetric data; fig. 15), except near the spur dikes on the right (west) bank (fig. 14; table 3). The channel bed between the left (east) bank and the spur dikes was filled with small dunes and ripples (fig. 14). Substantial scour holes were present downstream from the spur dikes on the right (west) side (fig. 14). Stone revetment was present on the left (east) bank throughout the reach (fig. 12). A localized deep scour hole downstream from the downstream spur dike on the right bank had a minimum channel-bed elevation of about 755 ft (table 3; fig. 14). As in previous surveys (Huizinga, 2012, 2015, 2020e), stone revetment was present on the left (east) bank throughout the reach (fig. 14).

The scour hole near pier 10 (figs. 14 and 1.2) had a minimum channel-bed elevation of 762 ft, about 4 ft above the bottom of the seal course elevation of 758 ft (table 7). The footing and seal course were evident at the upstream end of pier 9 in the map and shaded pier TIN images (figs. 14 and 1.2A–B). The cross section along the upstream bridge face indicates the channel bed is lower than the seal course at pier 9 (fig. 16; table 7), and a hazy area under the upstream end of the seal course in the shaded pier TIN of the right side (fig. 1.2D) further implies the seal course was undermined. The pile cap footing of upstream pier 8 also appears to be partly undermined in the cross section along the upstream bridge face (fig. 16; table 7), but the point data were too thin in this area to create a meaningful shaded pier TIN of the area. Information from bridge plans indicates that piers 9 and 10 are founded on shafts drilled into bedrock, with about 13 ft of bed material between bedrock near pier 9 and the bottom of the scour hole caused by the spur dike (figs. 14 and 16; table 7). Pier 8 is founded on steel H-piles driven to refusal on bedrock, also with about 13 ft of bed material between bedrock near pier 8 and the bottom of the scour hole caused by the spur dike (figs. 14 and 16; table 7). In modern construction, bridge substructural elements usually are pinned or socketed to bedrock (Brown and others, 2018; American Association of State Highway Transportation Officials, 2020), but exposure of usually buried substructural elements warrants special consideration and observation. Along the upstream bridge face, the surveyed bed from 2022 generally falls within the fluctuating range of elevations observed in previous multibeam surveys, except near pier 10 where the 2022 cross section most closely mimics the 2014 survey (fig. 16).

The difference between the survey on June 22, 2022, and the previous survey on July 16, 2018 (fig. 17), indicates about 98 percent of the joint area of interest had detectable change, which means only about 2 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Erosion appears dominant throughout the reach between 2018 and 2022 in the DoD, except localized areas of substantial deposition downstream from the spur dikes on the right (west) bank and minor to moderate deposition in the middle of the upstream channel (fig. 17). The average difference between the bathymetric surfaces was –1.83 ft (table 8), indicating moderate channel degradation between the 2018 and 2022 surveys. The net volume of cut in the reach from 2018 to 2022 was about 134,200 yd³, and the net volume of fill was about 46,400 yd³, resulting in a net loss of about 87,800 yd³ of sediment between 2018 and 2022 (table 8). The bed surface in the cross section from the 2022 survey along the upstream face of the bridge was below the 2018 survey section, except near pier 9 (fig. 16). The water-surface elevation in 2022 also was substantially lower than in 2018 (table 8; fig. 16). The frequency distribution of bed elevations in 2022 is similar in shape to the 2018 distribution, but with a higher percentage of cells at channel-bed elevations about 4–5 ft lower than 2018 (fig. 15). The area downstream from the spur dike on the upstream right bank showed substantial erosion of as much as 30 ft and deposition of as much as 20 ft (fig. 17), and the erosion likely affected the scour observed near pier 10 (figs. 16 and 17). The stone revetment on the left (east) bank showed localized signs of minor erosion and deposition, and the various spur dikes throughout the channel showed minor scour on one face and deposition on the other (fig. 17). As with previous DoDs, deposition or scour apparent on opposing faces of a feature likely results from minor horizontal positional variances between the surveys (fig. 6). The areas of apparent erosion of the stone revetment tend to be near the upper edges of the banks and may be the result of larger uncertainty in the outer beams in the 2022 survey, particularly near the water surface (fig. 5).

The difference between the survey on June 22, 2022, and the survey on June 4, 2014 (fig. 18), indicates about 60 percent of the joint area of interest had detectable change, which means about 40 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Minor erosion and deposition appear nearly balanced throughout the reach between 2014 and 2022 in the DoD, with deposition slightly more dominant and with localized substantial erosion and deposition near the ends and downstream from the spur dikes (fig. 18). However, the average difference between the bathymetric surfaces was +1.02 ft (table 8), indicating moderate channel aggradation between the 2014 and 2022 surveys, and the net gain of sediment between 2014 and 2024 was about 22,900 yd³ (table 8). The cross section from the 2022 survey along the upstream face of the bridge is most like the 2014 survey section, varying 5 to 10 ft above and below the 2014 section (fig. 16). The frequency distribution of bed elevations in 2022 appeared similar in shape to 2014 but shifted about 1–2 ft higher in channel-bed elevations than 2014 (fig. 15). Streamflow in 2022 was most similar to 2014 (table 8), which may account for the similarity in overall characteristics between the surveys (figs. 16 and 18). The stone revetment on the left (east) banks shows a similar area of erosion to the 2018 comparison, again indicating possibly higher uncertainties of the outer beams in the 2022 survey, but most of the left bank is within the bounds of uncertainty (fig. 18). A substantial part of the channel on the right (west) side was not surveyed in 2014 because of substantial floating debris (table 8). As with previous DoDs, deposition or scour apparent on opposing faces of pier 10 likely resulted from minor horizontal positional variances between the surveys (fig. 6).

The difference between the survey on June 22, 2022, and the earliest survey during flooding on July 14, 2011 (fig. 19), indicates about 85 percent of the joint area of interest had detectable change, which means about 15 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Erosion appears dominant between 2011 and 2022 in the DoD, with substantial erosion balanced with moderate to substantial deposition in the upstream channel and downstream from the spur dikes (fig. 19). The average difference between the bathymetric surfaces was –1.34 ft (table 8), indicating moderate degradation between the 2011 and 2022 surveys, with a net loss of sediment of about 55,800 yd³ (table 8). The frequency distribution of bed elevations in 2011 was very similar to 2022 (and the other surveys) but shifted about 2–3 ft higher in channel-bed elevations than 2022 (fig. 15). The stone revetment on the left (east) bank showed localized signs of minor scour and deposition, with the same area of moderate erosion as the other DoDs for this site (fig. 19). The piers and various spur dikes throughout the channel showed minor scour on one face and deposition on the other (fig. 19). As with previous DoDs, deposition or scour apparent on opposing faces of a feature likely resulted from minor horizontal positional variances between the surveys (fig. 6).

The vertically averaged velocity vectors indicate mostly uniform flow in the middle of the channel, with velocities ranging from about 3 to 8 ft/s (fig. 20). Local lower velocities and turbulence were observed downstream from the various spur dikes on the right (west) bank (fig. 20). Wake vortices were evident downstream from main channel pier 10 but were not pronounced and seemed to be no greater than the general nonuniformity of flow observed in the channel (fig. 20). Although aligned with primary flow in the channel, pier 10 is slightly skewed to approach flow because of the spur dike on the right (west) bank upstream, causing minor wake vortices and a defined deposition ridge downstream (fig. 20). Minor localized turbulence was present in all the sections (fig. 20).

Structure A5054 on Interstate 72 at Hannibal, Missouri

Structure A5054 (site 31; table 2) on Interstate 72 crosses the Mississippi River at RM 309.5 at Hannibal, Mo., north of St. Louis, Mo., and south of Quincy, Ill. (fig. 1). The site was surveyed on June 13, 2022, when the average water-surface elevation near the bridge, determined by the RTK GNSS tide solution, was 463.3 ft (table 3; fig. 21), and streamflow on the Mississippi River was measured at about 156,000 ft³/s during the survey (table 3).

The survey area was about 1,640 ft long and about 1,870 ft wide, extending from bank to bank in the main channel (fig. 21). The survey area extended about 700 ft upstream from the centerline of structure A5054 (fig. 21). The approximate channel-bed elevations ranged from about 428 to 447 ft for most of the surveyed area (5th to 95th percentile range of the bathymetric data; table 3; fig. 22), except near piers 6 through 8 and the thalweg of the channel along the base of the right (southwest) bank (fig. 21; table 3). The left side of the channel between the left (northeast) bank and the channel thalweg was filled with small and medium dune features with superimposed ripples. A spur dike on the left side downstream from the bridge was barely visible above the average bed nearby. As in previous surveys (Huizinga, 2015, 2020e), the right bank consisted of smooth rock or hardpan clay throughout the reach, with no discernable dune features (fig. 21).

The minor scour holes near the upstream end of piers 10 and 11 were difficult to discern from nearby dunes and ripples, and no hole was discernable near pier 5 on the right bank (figs. 21, 1.3A–D, 1.4G). Alternatively, piers 6 through 8 had well-defined, asymmetric scour holes indicative of being skewed to approach flow. Parts of the footing appear exposed for all the piers in the bathymetric map and in the shaded TIN images (figs. 21, 1.3, 1.4), and piers 6, 7, 9, and 10 seem to be surrounded by partial piles of riprap (figs. 21, 1.3, 1.4). Information from bridge plans indicates that piers 5 through 8 are founded on shafts drilled 15 to 28 ft into bedrock, having about 13 to 32 ft of bed material between the channel bottom and bedrock (table 7; fig. 23). Piers 9 through 11 are founded on steel H-piles driven to refusal on bedrock, having 54 ft of bed material between the channel bottom and bedrock at pier 9, 80 ft of bed material at pier 10, and 82 ft of bed material at pier 11 (fig. 23; table 7). The surveyed bed along the cross section generally was similar to previous multibeam surveys (fig. 23), but as much as 15 ft lower than previous surveys between the left bank and pier 10, and between pier 6 and 8. The cross section along the upstream bridge face indicates partial footing exposure at all the piers (fig. 23), and the exposure is readily apparent in the shaded TIN images of the piers (figs. 1.3, 1.4).

The difference between the survey on June 13, 2022, and the previous survey on July 18, 2018 (fig. 24), indicates about 86 percent of the joint area of interest had detectable change, which means about 14 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Erosion appears dominant throughout most of the reach between 2018 and 2022 in the DoD, except for localized areas of moderate deposition in the troughs of the medium dunes in the channel (fig. 24). The average difference between the bathymetric surfaces was –1.86 ft (table 8), indicating moderate channel degradation between the 2018 and 2022 surveys. The net volume of cut in the reach from 2018 to 2022 was about 205,500 yd³, and the net volume of fill was about 26,700 yd³, resulting in a net loss of about 178,800 yd³ of sediment between 2018 and 2022 (table 8). The frequency distribution of bed elevations in 2022 appears similar in shape to 2018, but with a higher percentage of cells at about 2-ft lower channel-bed elevations (fig. 22). The smooth rock or hardpan clay on the right (southwest) bank showed localized signs of minor erosion and deposition, and the left (northeast) bank appears shifted to eastward, resulting in substantial erosion along that bank upstream and downstream from the bridge (fig. 24). This movement of the left (northeast) bank also is evident in recent aerial imagery, which shows 5 to 15 ft of shift in the top of the bank location between 2012 and 2023 (Google Earth, 2012–23; https://earth.google.com/).

The difference between the survey on June 13, 2022, and the earliest survey on June 5, 2014 (fig. 25), indicates about 71 percent of the joint area of interest had detectable change, which means about 29 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Scour and deposition appear roughly balanced throughout the reach between 2014 and 2022 in the DoD, with scour and deposition alternating in the troughs of the small to medium dune features in the middle of the channel (fig. 25). The average difference between the bathymetric surfaces was –0.30 ft (table 8), indicating minor channel degradation between the 2014 and 2022 surveys, and the net loss of sediment between 2014 and 2022 was about 23,500 yd³ (table 8). The frequency distribution of bed elevations in 2022 is similar to 2014, with a slightly higher percentage of cells at channel-bed elevations above 435 ft (fig. 22). The smooth rock hardpan clay on the right (southwest) bank was mostly equivocal with localized signs of very minor deposition, and as noted with the 2018 comparison and confirmed with recent aerial imagery, the left (northeast) bank appears shifted to eastward, resulting in substantial erosion along that bank upstream and downstream from the bridge (fig. 25). As with previous DoDs, deposition or scour apparent on opposing faces of a feature likely results from minor horizontal positional variances between the surveys (fig. 6).

The vertically averaged velocity vectors indicate mostly uniform flow in the reach, but with localized flow disturbances in direction and magnitude in several of the sections (fig. 26). Velocities ranged from about 2 to 10 ft/s (fig. 26), with lower velocities and turbulence observed downstream from the piers and along the right (southwest) bank (fig. 26). All the piers were skewed to approach flow by 10 to 20 degrees (table 7), causing minor wake vortices and well-defined deposition ridges downstream (fig. 26). Additional flow disturbance may result from upwelling of flow caused by the numerous medium dune features present throughout the channel (Best, 2005; fig. 26).

Structure L0135 on State Highway 51 at Chester, Illinois

Structure L0135 (site 36; table 2) on State Highway 51 crosses the Mississippi River at RM 109.9 at Chester, Ill., southeast of and downstream from St. Louis, Mo. (fig. 1). The site was surveyed on June 14, 2022, when the average water-surface elevation of the river in the survey area, determined by the RTK GNSS tide solution, was 363.3 ft (table 3; fig. 27) and streamflow on the Mississippi River was about 339,000 ft³/s during the survey (table 3).

The survey area was about 1,640 ft and about 1,840 ft wide, extending across the active channel from the left (northeast) bank to the right (southwest) bank in the main channel (fig. 27). The upstream end of the survey area was about 655 ft upstream from the centerline of structure L0135, and piers 10 through 13 were in the water; however, pier 13 was in shallow water on the left bank (fig. 27). The channel-bed elevations ranged from about 316 to 347 ft for most of the surveyed area (5th to 95th percentile range of the bathymetric data; table 3; fig. 28), except near pier 11 and in the thalweg along the left side of the channel (fig. 27; table 3). A well-defined thalweg was present along the left (northeast) bank, about 15 to 20 ft deeper than the channel bed on the right side (fig. 27). A series of medium dunes were present in the channel thalweg and along the right side of the channel throughout the reach, along with numerous small dunes and ripples (fig. 27). As in previous surveys (Huizinga, 2015, 2020e), a bedrock outcrop was present on the left (northeast) bank, and a stone revetement was present on the right (southwest) bank (fig. 27).

Near pier 11, a substantial scour hole had a minimum channel-bed elevation of about 309 ft (table 7), which is about 17 ft below the average channel elevation upstream from the pier (figs. 27, 1.5D–E). Information from bridge plans indicates that pier 11 is founded on a caisson on bedrock, having about 43 ft of bed material between the bottom of the scour hole and bedrock (fig. 29; table 7); the top of the caisson is visible in the shaded TIN images (fig. 1.5D–E). Near pier 10, a moderate scour hole had a minimum channel-bed elevation of about 329 ft (table 7), which is about 8 ft below the average channel elevation upstream from the pier (figs. 27, 29, 1.5F–G). Pier 10 also is founded on a caisson on bedrock, having about 84 ft of bed material between the bottom of the scour hole and bedrock (fig. 29; table 7). Pier 12 is founded on footings on the exposed bedrock on the right bank, with no apparent scour hole nearby (figs. 27, 29, 1.5B–C; table 7). Pier 13 also is founded on footings on the exposed bedrock on the right bank and was near the surveyed water surface with no apparent scour hole nearby (figs. 27, 29, 1.5A; table 7). In modern construction, bridge substructural elements usually are pinned or socketed to bedrock (Brown and others, 2018; American Association of State Highway Transportation Officials, 2020), but full exposure of usually buried substructural elements warrants special consideration and observation, particularly at an older bridge such as structure L0135 (built in 1940) even when designed to be exposed.

The difference between the survey on June 14, 2022, and the previous survey on July 24, 2018 (fig. 30), indicates about 81 percent of the joint area of interest had detectable change, which means about 19 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Scour and deposition appear roughly balanced throughout the reach between 2018 and 2022 in the DoD, with apparent scour and deposition alternating in the troughs of the medium dune features in the thalweg and right side of the channel (fig. 30). The average difference between the bathymetric surfaces was –0.52 ft (table 8), indicating minor degradation of the reach between the 2018 and 2022 surveys. The net volume of cut in the reach from 2018 to 2022 was about 92,600 yd³, and the net volume of fill was about 46,100 yd³, resulting in a net loss of about 46,500 yd³ of sediment between 2018 and 2022 (table 8). The scour holes near piers 10 and 11 are shallower and slightly narrower in 2022 than in 2018 (figs. 29 and 30). The frequency distribution of bed elevations in 2022 appears very similar to 2018, with a slightly higher percentage of cells at the highest elevations than 2018 (fig. 28). The bedrock outcrop along the left (northeast) bank showed localized signs of minor scour, but most of the rock outcrop indicated changes within 1–3 ft of the 2018 elevation, or changes outside the 95-percent confidence interval (fig. 30). As with previous DoDs, deposition or scour apparent on opposing faces of a feature likely results from minor horizontal positional variances between the surveys (fig. 6).

The difference between the survey on June 14, 2022, and the earliest survey on June 9, 2014 (fig. 31), indicates about 70 percent of the joint area of interest had detectable change, which means about 30 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Deposition appears dominant throughout most of the reach between 2014 and 2022 in the DoD, with scour and deposition alternating in the troughs of the medium dune features in the thalweg and middle of the channel (fig. 31). The average difference between the bathymetric surfaces was +0.35 ft (table 8), indicating minor channel aggradation between the 2014 and 2022 surveys, and the net gain of sediment between 2014 and 2022 was about 27,300 yd³ (table 8). The scour holes near piers 10 and 11 are nearly identical in 2014 and 2022 (figs. 29 and 31). As with 2018, the frequency distribution of bed elevations in 2022 is very similar to 2014, with a slightly lower percentage of cells at elevations above 330 ft than in 2014 (fig. 28). Changes to the stone revetment along the right (southwest) bank is mostly equivocal, with localized signs of deposition downstream from the bridge (fig. 31). The bedrock outcrop on the left (northeast) bank showed localized signs of minor to moderate deposition (fig. 31); however, the differences near the rock outcrop would be most affected by minor horizontal positional variances between the surveys. As with previous DoDs, deposition or scour apparent on opposing faces of a feature likely results from minor horizontal positional variances between the surveys (fig. 6).

The vertically averaged velocity vectors indicate mostly uniform flow in the reach, but with localized flow disturbances in direction and magnitude in several of the sections (fig. 32). Velocities ranged from about 3 to 10 ft/s (fig. 32), with lower velocities and turbulence observed along the right (southwest) bank (fig. 32). Piers 11 and 12 were aligned with flow, as indicated by little to no turbulence observed downstream that can be directly attributed to these piers (fig. 32). Pier 10 had a flow angle of attack of about 5 degrees (table 7), evidenced with turbulence and ridge of sediment deposition downstream (fig. 32). Additional flow disturbance may result from upwelling of flow caused by the numerous medium dune features present throughout the channel (Best, 2005; fig. 32).

Structure A5076 on State Highway 34 at Cape Girardeau, Missouri

Structure A5076 (site 37; table 2) on State Highway 34 crosses the Mississippi River at RM 51.5 at Cape Girardeau, Mo., southeast of and downstream from Chester, Ill., and St. Louis, Mo (fig. 1). The site was surveyed on June 14, 2022, when the average water-surface elevation near the bridge, determined by the RTK GNSS tide solution, was 331.1 ft (table 3; fig. 33) and streamflow on the Mississippi River was about 335,000 ft³/s during the survey (table 3).

The survey area was about 1,640 ft long and about 1,970 ft wide (perpendicular to flow), extending across the active channel from the left (east) bank to the right (west) bank in the main channel (fig. 33). The upstream end of the survey area was about 730 ft upstream from the centerline of structure A5076 at pier 3, and piers 3 through 6 were in the water. Piers 2 and 7 were in very shallow water on the right and left banks, respectively, and were not surveyed. The channel-bed elevations ranged from about 276 to 318 ft for most of the surveyed area (5th to 95th percentile range of the bathymetric data; table 3; fig. 34), except in the well-defined channel thalweg along the right bank, and downstream from piers 3 and 4 (fig. 33; table 3). Numerous small to medium dunes and ripples were present throughout the channel. The area near the left bank upstream from the bridge was about 20 to 25 ft shallower than in the thalweg on the right side of the channel (fig. 33). As in previous surveys (Huizinga, 2015, 2020e), stone revetment was present on the right (west) bank (fig. 33).

As in previous surveys, piers 3 and 4 seemed to be partially surrounded by piles of riprap or rock (figs. 33, 1.6E–H). Scour downstream from the piers in 2022 reached a minimum elevation of 264 ft near pier 3 (table 7; fig. 33) and 270 near pier 4 (fig. 33). A poorly defined, minor scour hole difficult to discern from nearby ripples existed near pier 5 (figs. 33, 1.6D), having a minimum channel-bed elevation of about 300 ft (table 7). A minor scour hole also existed near pier 6 (figs. 33, 1.6A–B). The top of the footing was visible at pier 5, and the top of the distribution cap was visible at piers 3 and 4 (figs. 33, 1.6C–H). The minimum channel-bed elevation near pier 5 is about 15 ft above the elevation of the bottom of the pier seal course of 285.00 ft, and the minimum channel-bed elevation near pier 6 is about 20 ft above the elevation of the bottom of the pier seal course of 292.00 ft (fig. 35; table 7). Information from bridge plans indicates that piers 3 and 4 are founded on caissons on bedrock, having about 14 ft of bed material between the bottom of the scour hole and bedrock at pier 3, and 38 ft of material at pier 4 (fig. 35; table 7). Piers 5 through 7 are founded on shafts drilled as much as 22 ft into bedrock, having about 56 ft of bed material between the bottom of the scour hole and bedrock at pier 5, and about 67 ft of material at pier 6 (fig. 35; table 7). The exposed tops of the footing and distribution caps at piers 3 through 5 (figs. 33 and 35) might provide some mitigation to the scour holes at these piers as the downward flow of the horseshoe vortex at the pier column face is blunted by the footing (Arneson and others, 2012).

The difference between the survey on June 14, 2022, and the previous survey on July 25, 2018 (fig. 36), indicates about 83 percent of the joint area of interest had detectable change, which means about 17 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Erosion appears predominant throughout the reach between 2018 and 2022 in the DoD, except near the left (east) bank where deposition is more dominant (fig. 36). The average difference between the bathymetric surfaces was –1.79 ft (table 8), indicating moderate to substantial channel degradation between the 2018 and 2022 surveys. The net volume of cut in the reach from 2018 to 2022 was about 215,200 yd³, and the net volume of fill was about 42,300 yd³, resulting in a net loss of about 172,900 yd³ of sediment between 2018 and 2022 (table 8). The cross section from the 2022 survey along the upstream face of the bridge generally varies above and below the 2018 survey section (fig. 35). The frequency distribution of bed elevations in 2022 has a slightly wider elevation range than either the surveys in 2018 or 2014 (fig. 34). The stone revetment on the right (west) bank showed localized areas of deposition, but nearly all the indicated changes are within 1 ft of the 2018 elevation, or outside the 95-percent confidence interval (fig. 36). Areas of localized erosion of the stone revetment tend to be near the upper edges of the banks and may be the result of larger uncertainty in the outer beams in the 2022 survey, particularly near the water surface (refer to fig. 5 in “Uncertainty Estimation” section). The scour hole downstream from the spur dike on the left (east) bank has widened to the east since 2018, resulting in substantial erosion there (fig. 36).

The difference between the survey on June 14, 2022, and the earliest survey on June 10, 2014 (fig. 37), indicates about 71 percent of the joint area of interest had detectable change, which means about 29 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Erosion again appears predominant throughout the reach between 2014 and 2022 in the DoD, except near the left (east) bank throughout the reach, and localized areas on the right (west) bank downstream from the bridge where deposition is more dominant (fig. 37). The average difference between the bathymetric surfaces was –1.71 ft (table 8), similar to 2018 and again indicating moderate to substantial channel degradation between the 2014 and 2022 surveys. The net loss of sediment between 2014 and 2022 was about 142,900 yd³, which is less than but consistent with the net loss of sediment between 2018 and 2022 (table 8). As mentioned in the previous paragraph, the frequency distribution of bed elevations in 2022 has a slightly wider elevation range than either the survey in 2018 or 2014 (fig. 34). The stone revetment on the right (west) bank again showed localized areas of deposition (fig. 37); however, nearly all the indicated changes are within 1 ft of the 2014 elevation or are equivocal with uncertainty, and the apparent deposition likely results from minor horizontal positional variances between the surveys (fig. 6). The scour hole downstream from the spur dike on the left (east) bank had widened eastward between 2014 and 2022 (likely since 2018, refer to preceding paragraph), resulting in substantial erosion (fig. 37). Deposition of as much as 15 ft also was observed downstream from the tip of the spur dike (fig. 37). As with previous DoDs, deposition or scour apparent on opposing faces of a feature likely results from minor horizontal positional variances between the surveys (fig. 6).

The vertically averaged velocity vectors indicate mostly uniform flow throughout most of the reach, with localized moderate to substantial turbulence and flow reversals near the piers and the spur dike on the downstream left (east) bank (fig. 38). Velocities generally ranging from about 3 to 9 ft/s, except in these turbulent areas where velocities of as much as 12 ft/s were observed (fig. 38). Piers 3 and 4 were skewed to approach flow by about 30 degrees (table 7), causing substantial wake vortices with flow reversal and well-defined deposition ridges downstream (fig. 38).

Structure A1700 on Interstate 155 near Caruthersville, Missouri

Structure A1700 (site 38; table 2) on Interstate 155 crosses the lower Mississippi River at RM 838.9 (of the lower Mississippi River) near Caruthersville, Mo., in the southeastern corner of Missouri (fig. 1). The site was surveyed on June 15, 2022, when the average water-surface elevation of the river in the survey area, determined by the RTK GNSS tide solution, was 250.3 ft (table 3; fig. 39), and streamflow on the Mississippi River was about 468,000 ft³/s during the survey (table 3).

The survey area was about 1,640 ft long and about 2,710 ft wide, extending across the active channel from the left (southeast) bank to the right (northwest) bank in the main channel (fig. 39). The upstream end of the survey area was about 675 ft upstream from the centerline of structure A1700 (fig. 39), and piers 15 through 20 were in the water. Pier 21 was in shallow water on the left bank and was not surveyed. The channel-bed elevations ranged from about 176 to 224 ft for most of the surveyed area (5th to 95th percentile range of the bathymetric data; fig. 40; table 3). A deep thalweg was present along the left (southeast) bank with one very large dune feature, and medium to large dunes with numerous small dunes and ripples were detected throughout the channel (fig. 39). As in previous surveys (Huizinga, 2015, 2020e), a stone revetment was present on both banks near the bridge (fig. 39).

The riprap blankets near piers 18 through 20 seen in previous surveys were again evident in the 2022 survey (fig. 39). Furthermore, new riprap blankets appeared to have been placed around piers 15 through 17 since 2018 (figs. 39, 1.7F, 1.8), enhancing stone revetment already near pier 15 and replacing the random piles of riprap observed near pier 16 in previous surveys (refer to figs. 33, 1.8 in Huizinga [2020e]). The scour holes historically observed near the various channel piers were not present in the 2022 survey (table 7). The minimum channel-bed elevation observed at piers 19 and 20 was 6 to 8 ft below the average channel-bed elevation upstream from the pier (table 7), but the cross section along the upstream bridge face indicated there had not been much change near these piers since 2011 (fig. 41). However, the riprap blankets added since 2018 near piers 15 through 17 were evident in the 2022 cross section; furthermore, it appears the original riprap blanket near pier 18 was thickened since 2018, particularly on the left side (figs. 41, 1.7F). Information from bridge plans indicates piers 15 to 18 are founded on piles, and piers 19 to 21 are founded on caissons, all with an unknown depth to bedrock (fig. 41; table 7). The riprap blankets near piers 19 and 20 were installed at the time of bridge construction in 1973 and appear to effectively mitigate scour near these piers (fig. 41). The new or modified riprap blankets near piers 15 through 18 appear to cover the footings and seal courses for these piers (figs. 41, 1.8). Future surveys would be helpful to determine the effectiveness of the scour mitigation.

The difference between the survey on June 15, 2022, and the previous survey on July 26, 2018 (fig. 42), indicates about 90 percent of the joint area of interest had detectable change, which means about 10 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Erosion and deposition appear roughly balanced throughout most of the reach between 2018 and 2022 in the DoD (fig. 42). Substantial deposition of more than 20 ft was observed along the right (northwest) bank downstream from the bridge, with a corresponding area of as much as 20 ft of erosion adjacent to the left (southeast; fig. 42). The change in the channel-bed elevations from the new and modified riprap blankets near piers 15 through 18 is evident in the DoD, including the material added to the left side of the pre-existing blanket near pier 18 (fig. 42). The average difference between the bathymetric surfaces was –0.71 ft (table 8), indicating minor to moderate channel degradation between the 2018 and 2022 surveys. The net volume of cut in the reach from 2018 to 2022 was about 405,600 yd³, and the net volume of fill was about 305,600 yd³, resulting in a net loss of about 100,000 yd³ of sediment between 2018 and 2022 (table 8). The frequency distribution of bed elevations in 2022 is most similar to the 2018 distribution, but with a slightly higher percentage of channel-bed elevations in the ranges between 182 and 194 ft, and 208 and 212 ft (fig. 40). The cross section along the upstream bridge face for 2022 also is most like the 2018 cross section, except near the new riprap blankets near piers 15 through 18, which are substantially built up (fig. 41). There is some localized erosion between piers 17 and 18, as well as erosion between piers 15 and 16 that extends to the downstream end of the reach (figs. 41 and 42). The stone revetment on the banks showed localized areas of erosion and deposition, but nearly all the indicated changes are within 3 ft of the 2018 elevation, or outside the 95-percent confidence interval (fig. 42). The left bank showed predominantly erosion, whereas the right bank showed predominantly deposition (fig. 42); however, deposition or scour apparent on opposing faces of a feature (or opposite banks) may result from minor horizontal positional variances between the surveys (fig. 6).

The difference between the survey on June 15, 2022, and the survey on June 11, 2014 (fig. 43), indicates about 86 percent of the joint area of interest had detectable change, which means about 14 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Substantial erosion appears dominant throughout most of the reach between 2014 and 2022 in the DoD, with as much as 40 ft of erosion from the area near the trough of the very large dune in the thalweg near the bridge (fig. 43). As with 2018, the change in the channel-bed elevations from the new and modified riprap blankets near piers 15 through 18 is evident in the DoD, and there is additional deposition on the right side of the channel upstream from the bridge (fig. 43). Again, there is some localized erosion between piers 17 and 18, as well as erosion downstream from piers 15 and 16 that extends to the downstream end of the reach (figs. 41 and 43). The average difference between the bathymetric surfaces was –4.69 ft (table 8), indicating substantial channel degradation between the 2014 and 2022 surveys with a net loss of sediment of about 636,400 yd³ between 2014 and 2022, which was the largest net loss of material of all the sites surveyed in 2022 (table 8). The frequency distribution of bed elevations in 2014 had been similar to 2008, with a narrower range of elevations than 2022 (fig. 40). The stone revetment on the left (southeast) bank showed localized areas of deposition, but much of the difference is equivocal (fig. 43). Deposition of as much as 15 ft at the base of the upstream left bank and on the right bank upstream from the bridge likely resulted from material added in these areas after the 2014 survey; the left bank deposition had been noted in Huizinga (2020e; fig. 43). As with previous DoDs, deposition or scour apparent on opposing faces of a feature (such as the piers and sunken barges, fig. 43) likely results from minor horizontal positional variances between the surveys (fig. 6).

The difference between the survey on June 15, 2022, and the survey during flooding on May 5, 2011 (fig. 44), indicates about 89 percent of the joint area of interest had detectable change, which means about 11 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Substantial deposition appears dominant throughout most of the reach between 2011 and 2022 in the DoD, with as much as 30 ft of deposition in the thalweg on the left side of the channel and very localized areas of erosion near the trough of the very large dune near the bridge and on the right side of the channel downstream from the bridge, particularly downstream from piers 15 and 16 (fig. 44). The average difference between the 2011 and 2022 bathymetric surfaces was +5.83 ft (table 8), indicating substantial channel aggradation with a net gain of sediment between 2011 and 2022 of about 745,000 yd³ (table 8), which is the largest average difference and net gain between all the surveys in this study. However, the substantial gain in sediment is to be expected because structure A1700 is the downstream-most site surveyed in Missouri (downstream from the confluences of the Missouri and Ohio Rivers) and therefore subject to the largest streamflows, the largest streamflow fluctuations, and the most substantial sediment flux, as has historically been observed at this site (table 8). The frequency distribution of bed elevations in 2011 is unique, with a higher percentage of cells at elevations 10 to 20 ft lower than 2022 (fig. 40). The stone revetment on the banks showed localized areas of moderate erosion and deposition (fig. 44). As noted in the preceding paragraph, substantial deposition at the base of the upstream left bank and on the right bank upstream from the bridge likely results from material added in these areas after the 2014 survey; however, the erratic movement of the boat and substantial depths experienced during the flood conditions at this site in 2011 likely exacerbated horizontal positional variances between the surveys (refer to the “Uncertainty Estimation” section; fig. 6).

The difference between the survey on June 15, 2022, and the earliest survey on December 10, 2008 (fig. 45), indicates about 91 percent of the joint area of interest had detectable change, which means only about 9 percent of the differences in the joint area of interest are equivocal and within the bounds of uncertainty (table 8). Substantial erosion appears dominant throughout most of the reach between 2008 and 2022 in the DoD, with as much as 40 ft of erosion from the area near the trough of the very large dune in the thalweg near the bridge and erosion along the toe of the right bank downstream from piers 15 and 16 (fig. 45). The average difference between the 2008 and 2022 bathymetric surfaces was –5.47 ft (table 8), which was the largest negative average change and indicated substantial channel degradation with a net loss of sediment between 2008 and 2022 of about 636,300 yd³ (table 8). This average difference was very similar to 2014 and was the second largest net loss of material of all the sites surveyed in 2022. The frequency distribution of bed elevations in 2008 also was similar to 2014, with a narrower range of elevations than 2022 (fig. 40). The stone revetment on the left (southeast) bank showed localized areas of moderate erosion and deposition (fig. 45). As noted in the preceding paragraphs, substantial deposition at the base of the upstream left bank likely results from material added in that area after the 2014 survey; however, much of the apparent erosion and deposition likely results from minor horizontal positional variances between the surveys, particularly because an SBET correction was not used in this earliest survey (refer to the “Uncertainty Estimation” section; fig. 6).

The vertically averaged velocity vectors indicate substantial variability of flow throughout the channel at this site (fig. 46). Velocities ranged from about 2 to 10 ft/s, with locally lower velocities and flow reversal in numerous locations along the banks, downstream from the bank constrictions near the bridge, and downstream from the piers. The new riprap blankets near piers 15 through 17 appear to cause localized flow disturbances upstream, downstream, and between the piers (fig. 46). Additional flow disturbance may result from upwelling of flow caused by the numerous medium to large dune features present throughout the channel (Best, 2005; fig. 46).